CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Common Stock	Additional Paid in Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Beginning balance (in shares) at Dec. 31, 2018		41,389,524
Beginning balance at Dec. 31, 2018	$ 318,253	$ 4	$ 589,795	$ (1,529)	$ (270,017)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share based compensation	16,125		16,125
Issuance of common stock under the equity incentive plan and proceeds from exercise (in shares)		207,730
Issuance of common stock under the equity incentive plan and proceeds from exercise	944		944
Unrealized gain (loss) on debt securities	2,359			2,359
Foreign currency translation adjustments	342			342
Issuance of common stock from maturity of the 2019 convertible debt outstanding (in shares)		1,297,343
Issuance of common stock from maturity of the 2019 Convertible debt outstanding	22,590		22,590
ESPP stock purchase and expense (in shares)		63,804
ESPP stock purchase and expense	1,512		1,512
Net income (loss)	(116,168)				(116,168)
Ending balance (in shares) at Sep. 30, 2019		42,958,401
Ending balance at Sep. 30, 2019	245,957	$ 4	630,966	1,172	(386,185)
Beginning balance (in shares) at Jun. 30, 2019		42,899,318
Beginning balance at Jun. 30, 2019	277,089	$ 4	625,999	781	(349,695)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share based compensation	4,178		4,178
Issuance of common stock under the equity incentive plan and proceeds from exercise (in shares)		59,083
Issuance of common stock under the equity incentive plan and proceeds from exercise	627		627
Unrealized gain (loss) on debt securities	76			76
Foreign currency translation adjustments	315			315
ESPP stock purchase and expense	162		162
Net income (loss)	(36,490)				(36,490)
Ending balance (in shares) at Sep. 30, 2019		42,958,401
Ending balance at Sep. 30, 2019	245,957	$ 4	630,966	1,172	(386,185)
Beginning balance (in shares) at Dec. 31, 2019		43,088,921
Beginning balance at Dec. 31, 2019	221,196	$ 4	636,910	726	(416,444)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share based compensation	16,613		16,613
Issuance of common stock under the equity incentive plan and proceeds from exercise (in shares)		361,722
Issuance of common stock under the equity incentive plan and proceeds from exercise	1,345		1,345
Equity offering (in shares)		7,475,000
Equity offering	108,692	$ 1	108,691
Unrealized gain (loss) on debt securities	250			250
Foreign currency translation adjustments	(622)			(622)
ESPP stock purchase and expense (in shares)		97,544
ESPP stock purchase and expense	1,748		1,748
Net income (loss)	(47,809)				(47,809)
Ending balance (in shares) at Sep. 30, 2020		51,023,187
Ending balance at Sep. 30, 2020	301,413	$ 5	765,307	354	(464,253)
Beginning balance (in shares) at Jun. 30, 2020		50,902,874
Beginning balance at Jun. 30, 2020	318,645	$ 5	758,945	1,399	(441,704)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share based compensation	5,139		5,139
Issuance of common stock under the equity incentive plan and proceeds from exercise (in shares)		120,313
Issuance of common stock under the equity incentive plan and proceeds from exercise	914		914
Equity offering	48		48
Unrealized gain (loss) on debt securities	(479)			(479)
Foreign currency translation adjustments	(566)			(566)
ESPP stock purchase and expense	261		261
Net income (loss)	(22,549)				(22,549)
Ending balance (in shares) at Sep. 30, 2020		51,023,187
Ending balance at Sep. 30, 2020	$ 301,413	$ 5	$ 765,307	$ 354	$ (464,253)